This invention relates to electrolytic smelting of aluminum from aluminum ore, and is more particularly concerned with a device for breaking the solidified crust that forms on the molten electrolyte and for metering doses of aluminum ore into the electrolytic bath to replenish the material that has been separated as aluminum metal. The invention is more specifically directed to a combined crust breaker and ore dispenser tool that is installed on the aluminum smelting tank or pot, and which can be remotely actuated to break the electrolyte crust or to add aluminum ore material, as needed.
The smelting process produces aluminum from an alumina powder, dissolved in a molten electrolyte, such as cryolyte, and where electric current is applied to the molten electrolyte bath. This takes place in a large vessel, or pot, which is closed off at the top to control off gasses produced by electrolysis. The metallic aluminum yield is siphoned off, in liquid form. Alumina powder is added to replace the portion that has been reduced to metal, and the process is run on a continuous basis. The molten electrolyte tends to solidify at the top surface and form a solid crust. The crust blocks off the molten electrolyte from the alumina powder that is added, and so the smelting equipment must include means for breaking up this crust to feed new alumina into the pot, so that the process can continue. The formation of crust complicates the automated smelting process because there are high electric currents involved, and the electrical properties of the bath depend on the concentration of alumina. If alumina powder is simply dispensed by depositing it onto the crust, followed by breaking of the crust, the alumina tends to drop into the bath in uncontrolled quantities, which can cause large swings in alumina concentration, upsetting the electrical characteristics of the bath. Also, when the crust is broken and the chunks of solidified material remelt, this can also dilute the electrolyte concentration and cause unpredictable electrical behavior.
It is typical for a smelting pot to employ a number of pneumatically driven crust breaker devices, each of which has a vertical shaft that can be driven vertically. The shaft typically has a hammer or chisel formed at the lower end. When the shaft is driven downward, the chisel penetrates the crust and forms an opening in it that permits the alumina to pass down into the molten electrolyte below. There is also a dispenser cup on the device that fills from an ore hopper when the shaft is in the raised position, and which dispenses the ore from the cup when the shaft descends. Thus, with this type of crust breaker device, each cycle drives the chisel through the crust of the electrolyte, and also dispenses a predetermined quantity of alumina ore.
The tying of the ore dispensing operation with the crust breaking operation does limit the effectiveness of this tool. When only crust breaking is required because too much crust has formed, then each actuation of the tool will dispense a measured quantity of the alumina, whether it is needed or not, each time the chisel is driven through the crust. Likewise, if only ore dispensing is needed, each time that the ore is dispensed the chisel is driven down into the electrolyte. It has been found that the presence of the chisel in the molten electrolyte can change the electrical characteristics of the bath, and so consequently it has been desired for some time to find a way to separate the ore dispensing operation from the crust breaking operation. Also, the heating of the chisel from repeated plunging into the bath causes the bath to adhere to the chisel, reducing the efficiency of the operation.
Most smelting bath assemblies at present have positions on the top cover where these crust breakers are installed, and it is also desired that any replacement tool with separate ore dispensing and crust breaking operations should be sized to occupy those spaces so that retrofitting can be performed as inexpensively and as easily as possible.
One previously proposed device for feeding alumina into the elecrolyte bath and for breaking the crust when needed, is discussed in Gerphagnon et al. U.S. Pat. No. 4,437,964. This device has two separate tools situated side by side, one being a crust breaker plunger and the other being the alumina dispenser. Because there are two separate tools used to perform these functions, the tool does not easily fit into the same geometry as the standard crust breaker tool that it would replace. Also, the crust breaker and the dispenser require separate air cylinders, which complicates the construction and the control mechanism.
Another proposed crust breaker and ore dispensing device is discussed in Kissane U.S. Pat. No. 5,423,968. In this proposed device, a first air cylinder at the top of the shaft of the crust breaker is used to drive the hammer or chisel end into the crust when needed. A second air cylinder is formed around a lower part of the shaft to control the dispensing cup independently of the crust breaker plunger. That is, a slide cylinder is formed around the plunger shaft, to control the upper seal and lower seal of the dispensing cup. This unit does fit into the same geometry as a standard, combined crust breaker and dispenser, and is capable of performing independent dispensing and crust breaking operations. However, this device does have several drawbacks. The device of the Kissane patent requires replacing the entire dispensing unit, and does not use any of the parts that it replaces, so it is requires a significant added expense to install these units. Also, the Kissane device requires sliding parts at the lower end where there are an abundance of highly abrasive alumina dust and also highly corrosive process off gases. The main alumina valve allows flow of the alumina dust up through the slide mechanism, resulting in the actuator for the dispensing cup experiencing a high rate of wear. The high friction surface is also exposed to the highly corrosive environment, and so the mechanical parts of the dispensing device are exposed to contamination each time the ore material is discharged.
Thus, there remains a need for a simpler arrangement that allows the ore dispensing and crust breaking functions to be separated, but which employs as much of the standard crust breaker and dispenser as possible, and which avoids placing high friction critical surfaces in the places where there are significant levels of corrosives and/or abrasives.